Field of the Invention
The present invention relates to a method for indicating the remanent magnetization of an earth formation penetrated by a bore hole, even in the presence of high solar and atmospheric disturbance, so as to indicate the geologic age of a segment of the formation and to document stratigraphic boundaries occurring, say, during generation and migration of petroleum over a given span of geologic time, such as when accumulative traps were generated.
In the 1940's geologists and geochemists were attracted to a new hypothesis of petroleum origin and accumulation: early accumulation of hydrocarbons was thought to occur primarily from living organisms present in small amounts in most marine segments at the time of deposition. In the 1960's, geochemical evidence mounted that refuted the early-accumulation hypothesis as the principal mechanism fofr petroleum generation and accumulation. Such evidence included the fact that hydrocarbon molecules from 2 to 13 carbon atoms, i.e., C.sub.2 -C.sub.13, have been shown to be virtually absent in modern sediments. As such hydrocarbons comprise up to 25 percent of an average petroleum accumulation, the former must be formed by progressive alteration of immobile hydrocarbon precursors.
In analyzing an earth formation penetrated by a well bore regarding hydrocarbon potential via progressive alteration, not only should indicies of, say, geothermal gradients, migration quantity and direction be generated, but also the age of the strata of interest must be accurately determined.
However, geologic age-dating of the adjacent strata by conventional in-hole dating equipment not only is time-consuming and costly but also produces inaccurate results.
It is well known in carrying out geologic dating that the magnetization of a section of earth formation is associated with magnetic fields that are intrinsic to that body and act at a distance from it. Since the direction and strength of the fields can be identified, there can be gained an indication of both origin of the body and, hence its geologic age, assuming the field of interest can be accurately determined.
The following equation relating the magnetization of a given sample volume of an earth formation as a source of potential magnetic field is well known: EQU M = M.sub.R + KH.sub.o
where M equals the magnetization or magnetic dipole moment per unit volume of a sample;
M.sub.r equals the natural remanent magnetization of the sample; and PA1 Kh.sub.o equals the induced magnetization of the sample. PA1 (a) memory means for storing the magnetic field waveform as binary data; PA1 (b) means for digital filtering such binary data according to their frequency characteristics, such filtering means including associated circuitry for migrating the filtered data to a series of depth locations as a function of conductivity of the intervening formational strata, and PA1 (c) means for storing the resulting data as a function of depth.
In contrast with the above known functional relationship, and as discussed in the above-cited applications assigned to the same assignee as in the present application, the measurement of the intensity associated with the contribution of the natural remanent magnetization can be difficult to obtain, since measurements are carried out in the presence of the earth's field. Not only is the intensity of the anoamaly due to the remanent magnetization small in comparison with the induced magnetization, and its direction dependent upon the paleomagnetic origin of the sample as well as subsequent movement of the sample remote from the origin as a function of geologic time ("continental drift"), but also the earth's normal field can undergo rapid variations due to incidence of electromagnetic waves of solar and atmospheric origins.